1. Field of the Invention
The invention relates to a palladium colloid solution, a utilization of the solution for coating electrically non-conductive substrate surfaces with metallic coverings, and a printed circuit board produced by using the solution.
2. Brief Description of the Prior Art
Metallization of electrically non-conductive substrate surfaces is known per se. Electrolytic processes are used for metallizing printed circuit boards, plastics parts for decorative applications and also for functional purposes. To this end the substrate surfaces, after an appropriate pretreatment, are activated with a solution normally containing noble metal, and then metallized in a currentless manner. If necessary, further metal layers may be applied to this first metal layer by means of an electrolytic coating process.
The currentless metallization process however has considerable disadvantages (difficulty of monitoring the process in currentless metallizing baths, use of formaldehyde as a reducing agent, which is suspected of being carcinogenic, utilization of complex formers which are difficult to handle in terms of waste water disposal). For this purpose, efforts were made in the past to develop better processes. In some processes, the process step of currentless metallization needed to generate the electrical surface conductivity, is no longer necessary.
The electrical surface conductivity necessary for electrolytic metallization is in this case for example generated by means of intrinsically conductive organic polymers, these being applied as a thin layer to the surfaces. In DE 38 06 884 C1, DE 39 23 832 A1, :DE 39 31 003 A1, EP 0 206 133 A1 and EP 0 413 109 A2, there are described methods which use such polymer layers and subsequent metallization. In these methods the conductive form of the polymer is generated by an acidic or oxidizing subsequent treatment of the monomer adsorbed on the substrate surface, in which the monomer is polymerized. A disadvantage however is that the conductive polymer layer is applied not only to the non-conductive substrate surfaces, but is also deposited on metallic surfaces such for example as the copper surfaces on printed circuit boards, so that subsequently deposited metal layers do not adhere sufficiently securely on these metal surfaces.
In another variant of the method, the surfaces with the conductive organic polymer layer are coated after treatment with an activator containing noble metal (DE 42 11 152 C1).
In another alternative method, disclosed in the documents DE 39 39 676 C1, DE 41 12 462 A1, DE 42 27 836 A1and GB 22 43 833A, the conductive polymer layer is formed only on the non-conductive surfaces, the nonconductive monomer reacting in an acidic solution directly with the oxidation agent adsorbed on the non-conductive substrate surfaces. However, the monomer compounds used, from which the conductive polymer film is generated, are, as in the preceding variant of the method, relatively volatile, so that special precautionary measures have to be taken in order to protect working personnel.
Furthermore, as disclosed in U.S. Pat. No. 4,631,117, U.S. Pat. No. 4,874,477, DE 41 41 416 A1 and WO 92/19092, thin carbon layers may be applied to the substrate surfaces in order to generate the surface conductivity. In this case also the conductive layers form not only on the non-conductive areas of the substrate surfaces, but on metallic surface areas. Therefore the carbon layers must be removed from these areas before metallization. This requires an additional process step.
In a further alternative method, the first conductive layer required for electrolytic metallization is generated by treatment in solutions containing noble metal. Various alternative methods are described for this purpose.
According to U.S. Pat. No. 3,099,608, the substrate surfaces are firstly treated with a palladium/tin oxide-colloid solution and then with a solution by means of which the adsorbed tin compounds are again removed from the surfaces. The surfaces can then be metallized with a copper solution containing pyrophosphate.
According to DE 33 04 004 A1, the surfaces are likewise treated with a palladium/tin oxide-colloid solution and then with a solution for removing the tin compounds. Thereafter electrolytic metallization can take place.
According to DE 33 23 476 C2 the surfaces are also treated with a palladium/tin oxide-colloid solution and then with a solution by means of which adsorbed tin compounds are removed from the surfaces. The surfaces are metallized with copper baths with special organic compounds which enable the preferred separation of the surface areas coated with palladium colloid.
According to DE 37 41 459 C1 the substrate surfaces, after a catalytic activation, for example with a palladium/tin oxide-colloid solution, are treated with a solution containing nitrogenous compounds, for example polyvinylpyrrolidone, and are thereafter electrolytically metallized.
According to U.S. Pat. No. 4,790,913 and U.S. Pat. No. 4,891,069 the surfaces are treated with a palladium/tin oxide-colloid solution which additionally contains a stabilizer and a promoter, the hydrogen occurring during the electrolytic deposition being taken up by the absorbed palladium colloid and reinforcing the electrolytic metallization. Salts of aluminum, titanium, zirconium and hafnium and as promoter materials such as organic hydroxy, thiourea compounds, surface-active compounds, amino acids, polycarboxylic acids, and water-soluble polymers are used. Thereafter electrolytic metallization can take place.
According to U.S. Pat. No. 5,071,517, the surfaces are treated with a palladium/tin oxide-colloid solution and then with a weakly alkaline solution for removing the tin compounds. Thereafter electrolytic metallization can take place.
According to EP 0 398 019 A1 the substrate surfaces, before treatment with a solution containing metal, such for example as a palladium/tin oxide-colloid solution, are treated with a solution which contains a surface-active compound and which reinforces the absorption of metal.
The above named methods have the disadvantage that a sufficiently dense and pore-free metal coating can only be deposited, and a sufficiently high degree of adhesion of the deposited metal film on the metallic areas, for example on the copper surfaces of a printed circuit board, can only be achieved in a narrow range of process parameters chosen, which can scarcely be maintained during industrial application of the methods.
According to U.S. Pat. No. 3,984,290, in a further alternative method, there is deposited on the entire surface of the substrate a more electropositive (nobler) metal than the metal present on the substrate in specific areas. During metallization of printed circuit boards, for example, palladium or silver is applied to the copper surfaces and the non-conductive areas. Then the metal coating formed is treated with an etching solution, which attacks only the original metal surfaces (copper), so that the nobler metal film formed is again removed from the original metal surfaces. Thereafter electrolytic metallization can take place. According to a variant of the method the substrate surfaces are treated, in conjunction with coating with the nobler metal, with a solution containing chalcogen compounds.
In a further variant of the method, the substrate surfaces are firstly treated with a colloid solution containing a noble metal, and then with a solution containing chalcogenide compounds. In this way the adsorbed noble metal is converted into a highly conductive noble metal-chalcogenide compound which is stable against chemical influences. More typically, a palladium/tin oxide-colloid solution is used as an activator and sodium sulphide as a chalcogenide compound (U.S. Pat. No. 4,895,739, U.S. Pat. No. 4,952,286, U.S. Pat. No. 5,007,990, U.S. Pat. No. 5,017,742).
According to U.S. Pat. No. 4,810,333, the substrate surfaces are first treated with an alkaline permanganate solution. There then forms on the substrate surfaces a manganese dioxide layer by a reaction of the permanganate with the organic material of the substrate. This layer is subsequently treated with a chalcogenide solution and then with a noble metal solution. Thereafter electrolytic metallization is carried out.
The treatment with the chalcogenide solution however leads, on metallic areas of the substrate surfaces, to a formation of metallic chalcogenides, so that these metallic chalcogenide layers must be removed before the subsequent electrolytic metallization by an etching process from the metal surfaces, in order to obtain sufficient adhesion of the deposited metal on the metal substrate. The etching process must be carried out within narrow parameter limits, in order on the one hand not to damage the formed conductive layer, and on the other hand to remove the entire metallic chalcogenide layer from the metal surfaces.
According to U.S. Pat. No. 4,919,768, the surfaces to be coated are firstly treated with a solution containing tin (II) ions, then with a chalcogenide solution and thereafter with a solution of a noble metal, for example with palladium chloride. Then the pretreated substrate surfaces can be electrolytically metallized.
In this case also the metallic chalcogenide layer formed must be removed from the metallic areas of the substrate in an etching process. The same problems arise as in the first case mentioned.
In all processes in which colloidal noble metal solutions and in particular palladium/tin oxide-colloid solutions are used, there exists the problem that the colloidal solutions are oxidation-sensitive. This is due above all to the easy oxidation of the tin (II) compounds into tin (IV) compounds. Due to oxidation of the tin compounds, the stabilizing effect on the colloid solution of the tin (II) ions is lost, as tin (IV) ions do not have a stabilizing effect. In this way the colloid coagulates, and palladium is precipitated.
In addition, only a limited palladium concentration can be adsorbed on the substrate surfaces from palladium/tin oxide-colloid solutions. Therefore the capacity for metallization of palladium layers, which are adsorbed from these colloids, is less than that of colloid solutions stabilized with organic protective colloids.
Another noble metal colloid solution is described in DE 42 03 577 A1. In this case there are involved colloids of the noble metal oxides which, after adsorption on the non-conductive substrate surfaces, must be reduced by a further treatment step to the corresponding metals.
Noble metal solutions stabilized with organic protective colloids are less sensitive to oxidation than the palladium/tin oxide-colloid solutions mentioned above. In U.S. Pat. No. 4,004,051, U.S. Pat. No. 4,634,468, U.S. Pat. No. 4,652,311 and U.S. Pat. No. 4,725,314, such noble metal solutions are described for processes with currentless metallization baths, preferably with alkaline copper baths.
In a further alternative method for direct electrolytic metallization, the substrate surfaces according to DE 42 06 680 C2 are treated with noble metal colloid solutions stabilized with organic protective colloids instead of with tin compounds. In addition, in order to remove the adsorbed protective colloid compounds from the substrate surfaces, a solution is used which contains sulphur compounds with sulphur in the oxidation stage of +1 to +5. This method has the advantage that coating of the non-conductive surface areas with the noble metal colloid is successful without deposition of a disturbing layer on the metallic areas of the surfaces to be coated.
However, the noble metal colloid solutions stabilized with organic protective colloids are also oxidation-sensitive, as the colloidal noble metal reacts with atmospheric oxygen, which is carried into the solution. In an acidic solution, the colloidal palladium is transferred by oxidation into soluble palladium salts, so that the noble metal colloid is destroyed. The colloid solutions react particularly in the use of continuous installations used for manufacturing printed circuit boards and in which the treatment solutions are sprayed or splashed onto the horizontally guided printed circuit boards, with oxygen, as the solutions in this case are intensively moved, and they therefore come more intensively into contact with air than when a dipping process is used.